The present invention relates to input scanners as would be found, for example, in digital copiers, facsimile machines, and the like.
Certain types of familiar office equipment, such as copiers, or facsimile machines, accept images from hard-copy sources for recording and reproduction. Very often these original images originate on sheets of paper which are fed through an automatic xe2x80x9cdocument handlerxe2x80x9d which takes a stack of sheets and feeds them serially over a platen or other window, where the images are recorded. In a digital copier, facsimile machine, or scanner, the window is associated with one or more photosensitive semiconductor chips which convert the light reflected from the series of small areas on each sheet into video signals which are then digitized and converted to digital image data, which can then be retained in a memory. In the case of the standard xe2x80x9clight lensxe2x80x9d or xe2x80x9canalogxe2x80x9d copier, the light reflected from the series of sheets fed through the scanner is directed to a photoreceptor, as is well known in the art.
In one particular type of document handler, a type which is particularly useful for digital applications such as a digital copier or facsimile, the platen or window over which sheets are successively passed is essentially a narrow slit, through which only a relatively thin line of each sheet is xe2x80x9cviewedxe2x80x9d at a time, such as by the photosensitive semiconductor chip. In such a design, each sheet being fed through the document handler is passed at a known velocity over the window or platen while the photosensitive semiconductor chip records and outputs data over time. In other words, in such an arrangement, the photosensitive chip is exposed a series of very thin views of the sheet over time, and then circuitry associated with the chip in effect reassembles a large number of such slits to accumulate data descriptive of the entire image on the sheet. One example of a design of a document handler of this configuration is given in U.S. Pat. No. 5,339,139. Such a document handler which moves sheets at a constant velocity over a narrow platen or window is known as a xe2x80x9cconstant velocity transportxe2x80x9d or CVT.
One practical problem which has been noticed in applications of a CVT, particularly in regard to digital copiers and facsimile machines, is the fact that a small quantity of dirt which may attach to the small window or platen over which each sheet passes can cause a significant image defect in resulting digital images. Because a spot of dirt is stationary on the window, the image of the dirt on the window is repeatedly scanned by the photosensitive chip even as successive areas of one sheet move over the window. When a printout is made of the image thus recorded with the CVT, the print will have a straight streak on it corresponding to the position of the original spot of dirt.
The present invention is directed toward a technique for minimizing or eliminating the effect of small quantities of dirt on the window of a CVT in a hard copy scanner.
In the prior art, U.S. Pat. No. 5,214,470 discloses a method and apparatus for detecting the presence of dirt or abrasions on a document platen and preventing these defects from being reproduced as unwanted spots on output copies. The platen is first scanned without a document placed thereon and an electronic image is generated and stored which contains information on the location of a dirt spot or inherent defect such as etched marks on the platen. Upon the initiation of a scanning mode with a document placed on the platen, the scanned electronic document image is subjected to a logical XOR process to correlate the previously stored spot location with the same location on the document image. Once the spot location is determined, the spot is erased within the digital image data. Significantly, this patent does not disclose the special problems of dirt appearing on the window of a CVT or other automatic document handler.
U.S. Pat. No. 5,339,139 is cited as an example of a document feeding system, otherwise known as a constant velocity transport or document handler, as would be found, for instance, in a currently-available digital copier.
U.S. Pat. No. 5,467,410 discloses a software technique for quickly determining whether a binary input image originated as a blank page, submission of blank pages being a possible occurrence when accepting data from a device having an automatic document handler. The disclosure recognizes that a page image may contain vertical streaks that are not part of the original paper document. A threshold analysis typically allows a certain number of such streaks to be present without concluding that the page is not blank.
U.S. Pat. No. 5,644,409 discloses a system used in conjunction with a document handler. In particular, the patent discloses a system for compensating for dirt accumulating within the document handler, such dirt affecting the white reference value when the apparatus is calibrated.
According to the present invention, there is provided a method of operating a document handler for causing an image-bearing sheet to pass over a light-transmissive window in a process direction for recording by a photosensitive member, the photosensitive member being exposed to a portion of the image-bearing sheet through a scan line which is narrower than the window along the process direction. The scan line is caused to move parallel to the process direction relative to the window.